A counter-holder of this type is already known from SE 8903797-2 and is used together with drilling equipment during the dental operation for tightening the screw connection which secures the so-called spacer against an anchoring element, preferably a titanium screw, a so-called fixture, implanted in the bone. With this known counter-holder, controlled, mechanical tightening of a screw connection of a dental implant is possible without any torque being taken up by the bone, that is the screw (the fixture) remains unstressed during mounting itself.
As can be seen from FIG. 2 in SE 8903797-2, the counter-holder has a first, tubular part, the base portion of which is provided with an internal space adapted to the spacer so that the spacer is locked and cannot be rotated in relation to the tubular part after the tubular part has been slipped over the spacer. Accordingly, the internal space is in the form of a hexagonal recess in order to fit the cylindrical, hexagonal upper portion of the existing spacers. The internal hole through the tubular part has a diameter such that the screwdriver tool of the drill can rotate freely in this space.
The tubular part is rigidly connected to a fork, the base of which extends essentially at right angles to the center line of the tubular part and the shanks of which are essentially parallel to said center line. The fork is U-shaped and is positioned with one shank on each side of the handpiece of the drill. The shanks have such a length that they extend beyond the diameter of the handpiece. As a result of this, the tightening torque will be taken up by the shanks of the fork instead of stressing the spacer and thus the fixture.
When a predetermined screw tightening torque has been achieved, the handpiece of the tool offers resistance to one shank of the fork. The tightening torque gives rise to a reaction torque which is the same as the tightening torque but in the opposite direction, which means that the tissue which surrounds the implant remains unstressed.
When the predetermined screw tightening torque has been achieved, the tightening tool and the counter-holder are to be removed, which can sometimes be difficult since the handpiece of the tool bears against one shank of the fork with the force which has been built up in the system. When the tool and the counter-holder are to be removed, this force must be released and there is then a risk that the tool or the implant are still stressed inadmissibly.
Instead of the metal spacers which have mainly been used until now, and which are also described in the abovementioned Swedish patent, it is also previously known to use ceramic distance pieces which, like the earlier metal spacers, are secured against the fixture with the aid of a distance screw which is screwed down into a drilled hole in the fixture with the aid of the tightening tool. There are advantages in using ceramic distance pieces instead of the metal distance pieces which were mostly found on the market until now, mainly for aesthetic reasons because the ceramic material is more similar to the surrounding teeth than metal material, but also because the ceramic material can be prepared and more easily individually shaped so that the outer shape of the distance piece is adapted to the anatomical situation in each individual case.
But there are also disadvantages with the ceramic material, mainly in terms of its strength. The material is more brittle and can fracture, especially if too great stresses occur in the material during tightening against the fixture. There is therefore also a risk of fractures in the material when the tool and the counter-holder are to be removed after tightening as a result of the abovementioned "locking" of the tool against one shank of the fork.